1. Field of the Invention
The present invention relates to an image heating apparatus appropriate for use as a thermal fixing apparatus, mounted, for example, in a copier or a printer, and a heater employed for this apparatus. In particular, the present invention relates to an image heating apparatus having a flexible metallic sleeve and a heater employed for this apparatus.
2. Description of the Related Art
Most conventional copiers and printers of an electrophotographic type adopt, as fixing means, a thermal roller fixing system, of a contact heating type, that provides satisfactory heating efficiency and safety, or a system whereby power is not supplied to a thermal fixing apparatus in the standby state and power consumption is as greatly reduced as possible; specifically, a film heating system of an energy saving type is one wherein a thin film is arranged between a heater and a pressure roller, and the thermal fixing of a toner image to a recording medium is performed through the film. An example thermal heating method that uses the film heating system is proposed, for example, in Japanese Patent Laid-Open No. Sho 63-313182, No. Hei 2-157878, No. Hei 4-44075 and No. Hei 4-204980. The schematic configuration of such an example film heating system is shown in FIG. 9. As shown in FIG. 9, a fixing apparatus of a film heating type includes: a heating member (a heating body; hereinafter referred to as a heater) securely supported by a stay holder (a support body); a heat resistant thin film (hereinafter referred to as a fixing film) 3, the inner peripheral surface of which contacts the heater 2; and a elastic pressure roller 4 that, with the heater 2, grips the film 3 to form a nip portion (a fixing nip portion) having a predetermined nip width. The heater 2 is controlled so as to maintain a predetermined temperature while power is received. The fixing film 3 is a cylindrical member, an endless belt shaped member, or a finite web roll member, and by using a rotation force supplied by drive transmission means (not shown) or the pressure roller 4, the fixing film 3 closely contacts and slides across the heater 2 at the fixing nip portion, and is conveyed in the direction indicated by an arrow.
In a condition under which the heat output by the heater has been adjusted to provide the predetermined temperature and the fixing film 3 has been moved in the direction indicated by the arrow, the medium to be heated, a recording medium bearing an unfixed toner image, is fed between the fixing film 3 and the pressure roller 4 at the fixing nip portion. The recording medium, held closely in contact with the face of the fixing film 3, and the fixing film 3 are then conveyed through the fixing nip portion. At the fixing nip portion, the toner image is heated by the heater 2, through the fixing film 3, and is thermally fixed to the recording medium. The recording medium, having passed through the fixing nip portion and having, thereafter, been separated from the face of the fixing film 3, is conveyed away from the fixing nip portion.
The stay holder 1, a heat resistant plastic member, for example, is used to hold the heater 2 and to guide the fixing film 3. In order to minimize the friction when the fixing film slides across the stay holder 1 and the heater 2, grease having a high heat resistance is used to coat the outer faces of the heater 2 and the stay holder 1. The pressure roller 4 is made by forming, around a core 6, a silicon rubber layer or a sponge layer 7 made of foamed silicon rubber, and then by forming, on the layer 7, a tubular shaped releasing layer 8 made of PTFE, PFA or FEP, or by applying a releasing layer 8 as a coating.
The fixing film 3 is quite thin, i.e., 20 to 70 μm, so that the heater 2 can efficiently apply heat at the fixing nip portion to the recording medium that is to be heated. The fixing film 3 includes three layers: a film base layer, a conductive primer layer and a releasing layer, with the film base layer on the heater side and the releasing layer on the pressure roller side. The film base layer is a heat resistant, very flexible layer that is made of a heat resistant resin, such as insulating polyimide, polyamideimide or PEEK, or a metal such as SUS, and has a thickness of about 15 to 60 μm. Further, because of the presence of the film base layer, the mechanical strength, such as the tear strength, of the entire fixing film 3 is maintained. The conductive primer layer is a thin layer, about 2 to 6 μm thick, and is electrically grounded in order to prevent the entire fixing film 3 from becoming charged. The releasing layer is a layer for preventing toner offset relative to the entire fixing film 3, and is made by applying a coating of a fluorine resin, such as PFA, PTFE or FEP, having a satisfactory release property of about 5 to 15 μm. Furthermore, in order to reduce the charge on the surface of the fixing film 3 and to prevent electrostatic offset, a conductive material, for example, is made by mixing carbon black having a specific resistance of about 103 Ωcm to 106 Ωcm in the releasing layer.
A ceramic heating member is generally employed as the heater 2. For example, using screen printing, a heat generating resistance layer, such as silver palladium (Ag/Pd).Ta2N, is formed in the longitudinal direction (the direction perpendicular to the plane of paper) on the surface (the surface that does not face the fixing film 3) of an electrically insulating, aluminum nitride ceramic substrate having a superior thermal conductive property and a small thermal capacity, and in addition, a heat generating resistance layer formation face is covered with a thin glass protective layer. Further, a slide layer is formed on the face of the ceramic substrate that contacts the fixing film 3 to reduce the damage friction may cause to the fixing film 3. The slide layer that contacts the fixing film 3 is generally made of glass when the base layer of the fixing film 3 is formed of a resin, such as polyimide. When the base layer of the fixing film 3 is made of a metal such as SUS, however, the durability of the glass layer is reduced. Therefore, to provide for such an event, a method whereby the slide layer on the slide face of the heater 2 is formed of a resin, such as polyimide or polyamideimide, is disclosed in Japanese Patent Laid-Open Publication No. 2003-57978.
According to the ceramic heater 2, when power is supplied to the heat generating resistance layer, the heat generating resistance layer generates heat, and the temperature of the entire heater, including the ceramic substrate and the slide layer, is rapidly raised. The rise in the temperature of the heater 2 is detected by temperature detection means 5, located at the rear of the heater 2, and is fed back to a power controller (not shown). The power controller controls the power supplied to the heat generating resistance layer, so that at the heater 2 a substantially predetermined temperature (a fixing temperature) is constantly detected by the temperature detection means 5. This control process enables the heater 2 to maintain a predetermined fixing temperature.
To increase the processing capability of an image forming apparatus, the heating efficiency of a fixing apparatus must also be increased. And in order to efficiently transmit heat generated by the heater to a recording medium, the conduction of heat by the base layer of the fixing film must be improved. For a resin fixing film, heat conduction can be improved by mixing heat conductive filler into the resin. However, when too large an amount of heat conductive filler is mixed into the resin, the tear strength of the fixing film is reduced and tearing of the film will occur. Thus, in order to eliminate the heat conduction and tear strength problems, a proposed fixing film is one for which the base layer is made of metal. When a metal fixing film is employed, as disclosed in Japanese Patent Laid-Open Publication No. 2003-57978, it is preferable that the slide layer of the heater be made of a resin such as polyimide.
It has been found, however, that when coping with an increase in the processing speed of an image forming apparatus, merely making the slide layer of the heater of a resin such as polyimide is not sufficient. Means for increasing the processing capability of the fixing apparatus can include the application of an increased pressurizing force at the fixing nip or the raising the temperature of the heater during the fixing process. However, increasing the pressuring force and raising the temperature of the heater both tend to accelerate the abrasion of the slide layer of the heater. As the slide layer of the heater is worn down by abrasion, particles removed from the slide layer mix with the grease between the surface of the heater and the metallic sleeve. As a result, the desired viscosity and smoothness of the grease is lost, the resistance produced by friction is increased, and the drive torque becomes greater. When the drive torque is increased, it is difficult to rotate the fixing film at high speed, and the processing capability of the fixing apparatus can not be improved. And when a thick slide layer is formed, although the durability of the slide layer is increased, the heat generated by the heater is not easily transmitted to the nip portion. Thus, the method employed to increase the thickness of the slide layer is also not acceptable.